Long Term Evolution (LTE), a radio access technology standardized by the 3rd Generation Partnership Project (3GPP), is based on orthogonal frequency division multiplexing (OFDM) in the downlink and single-carrier frequency domain multiple access (SC-FDMA) in the uplink. This use of OFDM and SC-FDMA divides transmission resources into time resources and frequency resources. Time resources are divided into subframes that are each 1 millisecond (ms) in duration. Each subframe is in turn generally divided into 12 or 14 slots, each of which is occupied by one OFDM or SC-FDMA symbol. Frequency resources in each subframe are divided into subcarriers. The combination of a particular slot at a particular subcarrier is referred to as a resource element (RE). The subframe can thus divide transmission resources into a plurality of REs. The REs can be organized into resource element groups (REGs) and physical resource blocks (PRBs). Each REG includes consecutive REs (e.g., 4 consecutive REs), while a PRB includes, for example, 72 REs (6 slots×12 subcarriers) or 84 REs (7 slots×12 subcarriers).
A subframe may also be divided into a control region and a data region. The control region may include, for example, 3 slots that carry the Physical Downlink Control Channels (PDCCHs). The PDCCHs are used to carry downlink control information (DCI) messages. Each PDCCH may be allocated transmission resources in units of control channel elements (CCEs). Each CCE includes, for example, 9 consecutive REGs. An aggregation level (L) indicates how many contiguous CCEs (also referred to as consecutive CCEs) are allocated to a PDCCH. Example aggregation levels include 1, 2, 4, and 8, where 8 may be a predetermined maximum number of consecutive CCEs that can be allocated to a DCI message in a PDCCH. An aggregation level of 2, for example, indicates that a PDCCH is allocated 2 consecutive CCEs.
Each PDCCH generally carries one DCI message. The DCI message may indicate, to a particular wireless communication device (WCD) receiving the subframe, which PRBs (if any) in the data region of the subframe are intended for that WCD. For instance, a base station (e.g., evolved node B (eNB)) may include in one subframe data for different WCDs (e.g., UEs). For each of these WCDs, the base station may include in the subframe a DCI message that indicates which PRBs in the subframe are intended for the WCD.
Different DCI formats exist for packing formatting information into a DCI message. Example formats include DCI formats 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 3, 3A and 4. DCI formats 0, 3, and 3A are used for granting uplink (UL) transmission resources to WCDs. DCI formats 1, 1A, 1B, 1C, 2, 2A, 2B, and 2C are used for assigning downlink (DL) resources to WCDs (see 3GPP 36.212, section 5.3.3). Different formats may provide for different DCI message sizes. A DCI message with a DCI format of 2 may, in one example, correspond to a bigger size than a DCI message with a DCI format of 1, though DCI message sizes may depend on a subframe's bandwidth. Some DCI formats may correspond to the same size. For example, a DCI message with a DCI format of 1A may have the same size as a DCI message with a DCI format of 0, 3, or 3A.
A DCI format such as 2A may support a spatial multiplexing scheme that uses multiple input/multiple output (MIMO) techniques to transmit different layers (e.g., streams) of data on different transmitters toward different receivers. For instance, a base station may use a transmission rank of 2 to transmit two different streams of data on two respective antennas toward two receivers of a WCD. Further, a DCI format such as 2A may allow physical resource blocks (PRBs) assigned in the DCI message to be non-contiguous.
What is desired is a method and apparatus for selecting a DCI format in which to send a DCI message in a DL subframe.